I disagree in that Generals don't seem useless and squishy to me. They do, however, generally seem rather bland. They're generally hard to kill enemies with okay stats and a lot of passive / support abilities. Their minions are nothing but creeps with better stats and a facelift.
Erebus, I think, is the exception. He does seem pretty interesting to me, probably the most unique out of any of the Demigods, actually.
I look at Oak, QoT, and Sedna, however, and just see pesky, persistent opponents that are about as fun to play as they are to play against, which is to say: not really fun and not really annoying, just "eh."
I think you're probably right that a team of Generals versus a team of Assassins will tend to lose, and I think that's because Generals have been cast in a support role. That makes sense, given that they're supposed to be support their minions. I think the problem may be, however, that their minions are really just support too.